Magnetoresistive Random Access Memory (MRAM) has a read function based on a tunneling magnetoresistive (TMR) effect in a MTJ stack wherein a tunnel barrier is formed between a free layer and a reference layer. The free layer serves as a sensing layer by switching the direction of its magnetic moment in response to external fields (media field) while the reference layer has a fixed magnetic moment. The electrical resistance through the tunnel barrier (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby provides an electrical signal that is representative of the magnetic state in the free layer. In MRAM, the MTJ is formed between a top conductor (electrode) and bottom conductor. When a current is passed through the MTJ, a lower resistance is detected when the magnetization directions of the free and reference layers are parallel (“0” memory state), and a higher resistance is noted when they are anti-parallel (“1” memory state). The TMR ratio is dR/R where R is the minimum resistance of the MTJ, and dR is the difference between the lower and higher resistance values. The tunnel barrier is typically about 10 Angstroms thick so that a current through the tunnel barrier can be established by a quantum mechanical tunneling of conduction electrons.
Another version of MRAM that relies on a TMR effect, and is referred to as a spintronic device that involves spin polarized current, is called spin-transfer torque (STT) MRAM and is described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996). J-G. Zhu et al. has described another spintronic device called a spin transfer oscillator (STO) in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry.
MTJ elements wherein one or both of the free layer and reference layer have perpendicular magnetic anisotropy (PMA) are preferred over their counterparts that employ in-plane anisotropy because the former has an advantage in a lower writing current for the same thermal stability, and better scalability. In MTJs with PMA, the free layer has two preferred magnetization orientations that are perpendicular to the physical plane of the layer. Without external influence, the magnetization or magnetic moment of the free layer will align to one of the preferred two directions, representing information “1” or “0” in the binary system. For memory applications, the free layer magnetization direction is expected to be maintained during a read operation and idle, but change to the opposite direction during a write operation if the new information to store differs from its current memory state. The ability to maintain free layer magnetization direction during an idle period is called data retention or thermal stability. The level of stability required is typically related to the memory application. A typical non-volatile memory device may require thermal stability at 125° C. for about 10 years.
CoFeB or the like is commonly used as the free layer (FL) and MgO is selected as the tunnel barrier to generate PMA along the FL/MgO interface. However, the physical shape of the free layer having a lateral dimension more than ten times the thickness tends to induce in-plane anisotropy. If the in-plane anisotropy is greater than the PMA component, the FL magnetization direction will be in-plane. As FL thickness (volume) increases, a greater portion of the free layer is away from the FL/tunnel barrier interface. A thicker free layer means a higher in-plane magnetic moment and a reduction in PMA and coercivity. Usually, free layer thickness must be maintained below 20 to 25 Angstroms to realize a condition where the PMA component is greater than the in-plane anisotropy in the magnetic layer.
In a memory element such as STT-MRAM, the current needed to change the magnetic orientation of a magnetic region (free layer) is proportional to the net polarization of the current, the volume, magnetization, Gilbert damping constant, and anisotropy field of the magnetic region to be affected. The critical current (ic) required to perform such a change in magnetization is given in equation (1):
                              i          c                =                                            α              ⁢                                                          ⁢              e              ⁢                                                          ⁢              V              ⁢                                                          ⁢              M              ⁢                                                          ⁢              s                                      g              ⁢                                                          ⁢                              h                _                                              ⁡                      [                                          H                                  k                                      eff                    ,                    ∥                                                              +                                                1                  2                                ⁢                                  H                                                            k                      eff                                        ,                    ⊥                                                                        ]                                              (        1        )            where e is the electron charge, α is a Gilbert damping constant, Ms is the saturation magnetization of the free layer, h is the reduced Plank's constant, g is the gyromagnetic ratio, Hkeff,∥, is the in-plane anisotropy field, and Hkeff,⊥ is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer. For most applications, the spin polarized current must be as small as possible. When the free layer has a magnetization direction perpendicular to the film plane, the critical current needed to switch the magnetic element is directly proportional to the perpendicular anisotropy field as indicated in equation (2):
                              i          c                =                              α            ⁢                                                  ⁢            e            ⁢                                                  ⁢            M            ⁢                                                  ⁢            s            ⁢                                                  ⁢            V            ⁢                                                  ⁢                          H                                                k                  eff                                ,                ⊥                                                          g            ⁢                                                  ⁢                          h              _                                                          (        2        )            
Thermal stability is a function of the perpendicular anisotropy field as shown in equation (3) where kB is the Boltzmann constant and T is the temperature:
                    Δ        =                                            M              S                        ⁢                                                  ⁢            V            ⁢                                                  ⁢                          H                                                k                  eff                                ,                ⊥                                                          2            ⁢                                                  ⁢                          k              B                        ⁢            T                                              (        3        )            
and the perpendicular anisotropy field of a magnetic layer is expressed in equation (4) as:
                              H                                    k              eff                        ,            ⊥                          =                                            -              4                        ⁢                                                  ⁢            π            ⁢                                                  ⁢                          M              s                                +                                    2              ⁢                                                          ⁢                              K                U                                  ⊥                                      ,                    s                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              χ              ,              ⊥                                                          (        4        )            where Ms is the saturation magnetization, d is the thickness of the magnetic layer, Hk,χ,⊥ is the crystalline anisotropy field in the perpendicular direction, and KU⊥,s is the surface perpendicular anisotropy of the top and bottom surfaces of the magnetic layer. The shape anisotropy field is represented by the term (−4πMs).
In order to improve thermal stability by increasing the value KU⊥,s, a second free layer/metal oxide interface is commonly introduced on a side of the free layer opposite to the tunnel barrier. The metal oxide may be another MgO layer and is often called a cap layer or a Hk enhancing layer. Thus, a MgO/FL/MgO stack will substantially increase total PMA in the free layer thereby allowing a thicker free layer and higher thermal stability. The cap layer typically contacts an uppermost MTJ layer called a hard mask, which in turn connects to a top electrode and through a top electrode array to complementary-metal-oxide-semiconductor (CMOS) units in a memory chip. The hard mask generally has a thickness in the hundreds of Angstroms, and is comprised of a metal such as Ta, Ru, Mo, MnPt, and their oxides and nitrides as required for conventional techniques in manufacturing integrated circuits. The remainder of the MTJ typically has a thickness of about 100 Angstroms, which means the volume of hard mask material is much greater than that of the other MTJ layers.
Since MTJ elements are implemented in CMOS devices, the MTJ must be able to withstand annealing temperatures up to about 400° C. for 30 minutes that are commonly applied to improve the quality of the CMOS units for semiconductor purposes. In most cases, this temperature exceeds the optimum temperature for best magnetic performance in the MTJ or MRAM. MTJs are usually annealed in the 300-330° C. degree range to obtain the desired magnetic properties.
One problem resulting from high temperature anneal around 400° C. is the diffusion of hard mask materials into the MTJ. Although a fully oxidized MgO cap layer is a good diffusion blocking material, the resistance×area (RA) product of fully oxidized MgO is quite high and adds a large series resistance to the MTJ that in turn undesirably reduces the signal difference between reading out “0” and “1” information stored in the device. Therefore, it is practical to employ only partially oxidized MgO cap layers less than 10 Angstroms thick. Unfortunately, hard mask materials are heavier than Mg and do not bind well with Mg in the partially oxidized MgO cap thereby enabling easier diffusion of heavier metals to the free layer which then degrades the capacity of the FL/cap layer interface to promote PMA in the free layer. As a result, free layer PMA is reduced and thermal stability is less compared with a condition where the MTJ is annealed only to 330° C., for example. Free layer coercivity is also less after high temperature annealing to around 400° C. than after 300-330° C. annealing. However, it is an important requirement to maintain coercivity after high temperature processing.
Thus, there is a significant challenge to increase PMA and enhance thermal stability of free layers to improve the performance of MTJs at elevated temperatures typical of back end of line (BEOL) semiconductor processes. Current technology fails to provide high He and thermal stability in a free layer with PMA character that will withstand high temperature processing up to at least 400° C., which is required in semiconductor fabrication methods. Therefore, an improved MTJ stack is needed to enable a free layer with thermal stability to at least 400° C., and that exhibits PMA for optimum magnetic memory performance.